Silicon carbine based porous material and method for preparation thereof, and honeycomb structure

ABSTRACT

A silicon carbide based porous material ( 1 ) containing silicon carbide particles ( 2 ) as an aggregate and metallic silicon ( 3 ) as a bonding material and having a number of pores ( 5 ) formed by them, characterized in that it has an oxide phase ( 4 ) in at least a part of the pore ( 5 ), and the oxide phase ( 4 ) contains respective oxides of silicon, aluminum and an alkaline earth metal and contains substantially no alkaline earth metal silicate crystal phase; a method for producing the above porous material; and a honeycomb structure comprising the silicon carbide based porous material. The above porous material is capable of effectively inhibiting the corrosion by an acid (especially acetic acid) used in the operation of carrying a catalyst, that is, is improved in the resistance to an acid.

TECHNICAL FIELD

The present invention relates to a silicon carbide porous body, a methodof manufacturing the same, and a honeycomb structure. More particularly,the present invention relates to a silicon carbide porous body which iseffectively inhibited from being corroded by an acid (particularlyacetic acid) used for loading a catalyst on the body to exhibit improvedacid resistance, a method of manufacturing the same, and a honeycombstructure including the silicon carbide porous body.

BACKGROUND ART

As a filter for trapping and removing particulate matter contained indust-containing fluid such as exhaust gas discharged from a dieselengine (diesel particulate filter (DPF)), or a catalyst carrier forsupporting a catalyst component for purifying a toxic substancecontained in exhaust gas, a porous honeycomb structure including cellpartition walls (ribs), which form a group of adjacent cells, and ahoneycomb outer wall, which encloses and holds the outermostcircumferential cells located at the outer circumference of the cellcomposite, has been widely used. As a material for the honeycombstructure, refractory silicon carbide (SiC) has been used.

A DPF using a regeneration method in which an oxidizing catalyst isloaded on a DPF and deposited particulates are oxidized and burned toeffect continuous regeneration (catalytic regeneration DPF) has alsobeen developed.

As such a honeycomb structure, a porous silicon carbide catalyst carrierhaving a honeycomb structure obtained by forming silicon carbide havinga predetermined specific surface area and containing impurities as astarting material into a desired shape, drying the formed product, andfiring the dried product in the temperature range of 1600 to 2200° C.has been disclosed (see JP-A-6-182228, for example).

In the sintering mode (necking) of the catalyst carrier disclosed inJP-A-6-182228 utilizing the recrystallization reaction of siliconcarbide particles, the silicon carbide component is evaporated from thesurface of the silicon carbide particles and condenses at the contactsection (neck section) between the particles, whereby the neck sectionis grown to obtain a bonding state. However, since an extremely highfiring temperature is required to evaporate the silicon carbide, cost isincreased. Moreover, since it is necessary to fire the material having ahigh coefficient of thermal expansion at a high temperature, firingyield is decreased.

In the case of manufacturing a filter having a high porosity(particularly a filter having a porosity of 50% or more) by sinteringutilizing the recrystallization reaction of the silicon carbideparticles, the growth of the neck section is hindered since thesintering mechanism does not sufficiently function, whereby the strengthof the filter is decreased.

As a related-art technology for solving these problems, a poroushoneycomb structure including refractory particles (particularly siliconcarbide) as an aggregate and metal silicon and a method of manufacturingthe same have been disclosed (see JP-A-2002-201082, for example).According to such a manufacturing method, a porous honeycomb structurecan be inexpensively manufactured at a relatively low firingtemperature, and the resulting porous honeycomb structure hascharacteristics such as a high porosity, high thermal conductivity, andhigh strength.

In order to improve the strength and the oxidation resistance (duringabnormal combustion or the like) of the honeycomb structure, atechnology of forming an oxygen-containing phase on the surfaces ofsilicon carbide and metal silicon by using a method of (1) oxidizing asilicon carbide raw material and metal silicon in air in advance oroxidizing the materials in a calcinating stage, (2) subjecting theproduct obtained after firing to a heat treatment in anoxygen-containing atmosphere, (3) coating the surface of the honeycombstructure using a solution containing silicon and oxygen, or the likehas been proposed (see JP-A-2002-154882, for example).

In order to further improve the strength of the honeycomb structure incomparison with the above-mentioned related-art technologies, there hasbeen proposed a method of adding an alkaline earth metal (calcium orstrontium) having an eutectic point of 1200 to 1600° C. to an oxide film(silicon dioxide) on the surfaces of silicon carbide as an aggregate andmetal silicon, and melting and removing the oxide film to improvewettability between the silicon carbide and the metal silicon, therebythickening the joint section between the silicon carbide and the metalsilicon (Japanese Patent Application No. 2002-61989). According to thismethod, a honeycomb structure having excellent strength can be obtained.However, the addition of calcium causes formation of a calcium silicate(alkaline earth metal silicate) crystal phase in the oxide phase afterfiring and dissolved in acetic acid used when causing the honeycombstructure to carry a catalyst, whereby a solution (solution in whichcatalyst is dissolved in acid) is contaminated. This problem was solvedby replacing calcium with strontium. However, a strontium silicate(alkaline earth metal silicate) crystal phase is formed in the samemanner as in the case of using calcium, depending the amount ofstrontium added and the firing conditions, and dissolved in acetic acid,resulting in poor acid (acetic acid) resistance.

DISCLOSURE OF THE INVENTION

The present invention provides a silicon carbide porous body which iseffectively inhibited from being corroded by an acid (particularlyacetic acid) used for loading a catalyst on the body to exhibit improvedacid resistance, a method of manufacturing the same, and a honeycombstructure including the silicon carbide porous body.

According to the present invention, the following silicon carbide porousbody, a method of manufacturing the same, and a honeycomb structure canbe provided.

[1] A silicon carbide porous body, in which silicon carbide particles asan aggregate and metal silicon as a bonding material are bonded so thatpores are formed between the silicon carbide particles, the siliconcarbide porous body comprising an oxide phase in at least a part of eachpore, wherein the oxide phase includes oxides of silicon, aluminum, andan alkaline earth metal, and the oxide phase does not substantiallyinclude an alkaline earth metal silicate crystal phase.

[2] The silicon carbide porous body as defined in [1], wherein the oxidephase is provided on a surface of the silicon carbide particles and/or asurface of the metal silicon.

[3] The silicon carbide porous body as defined in [1] or [2], whereinthe oxide of silicon is silicon dioxide (SiO₂), the oxide of aluminum isdialuminum trioxide (Al₂O₃), and the oxide of an alkaline earth metal iscalcium oxide (CaO) or strontium oxide (SrO).

[4] The silicon carbide porous body as defined in [3], wherein the oxidephase includes the dialuminum trioxide in an amount of 5.0 to 50.0 mol %of the entire oxide phase in molar ratio.

[5] The silicon carbide porous body as defined in any of [1] to [4],wherein the oxide phase includes silicon dioxide, dialuminum trioxide,and the oxide of an alkaline earth metal, the oxide phase being anamorphous phase or a crystal phase including dialuminum trioxide in acrystal structure.

[6] The silicon carbide porous body as defined in [5], wherein thecrystal phase includes cordierite, anorthite, or strontium feldspar(SrAl₂Si₂O₈).

[7] A honeycomb structure, comprising the silicon carbide porous body asdefined in any of [1] to [6].

[8] A method of manufacturing a silicon carbide porous body, the methodcomprising: adding compounds containing silicon, aluminum, and analkaline earth metal to silicon carbide particles and metal silicon toobtain a raw material, forming the resulting raw material into apredetermined shape, and calcinating and firing the resulting formedproduct to obtain a porous body including an oxide phase includingoxides of silicon, aluminum, and an alkaline earth metal on at least apart of a surface of the silicon carbide particles and/or the metalsilicon and having a content of dialuminum trioxide (Al₂O₃) of 5.0 to50.0 mol % of the entire oxide phase in molar ratio.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an enlarged cross-sectional diagram of a part of a crosssection cut at an arbitrary surface showing one embodiment of a siliconcarbide porous body of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Since the silicon carbide porous body of the present invention includesthe oxide phase in at least a part of each pore, and the oxide phaseincludes oxides of silicon, aluminum and an alkaline earth metal, andthe oxide phase does not substantially include an alkaline earth metalsilicate crystal phase, the oxide phase is not dissolved in an acid(particularly acetic acid) used for loading a catalyst on the body toexhibit improved acid resistance, whereby contamination of a solution(solution in which catalyst is dissolved in acid) used for loading acatalyst on the body is inhibited. Since the oxide film (SiO₂ or thelike) on the surfaces of the silicon carbide particles and the metalsilicon can be easily melted and removed by decreasing the eutecticpoint of the oxide phase by making the oxide phase a ternary oxide phasemade up of oxides of silicon, aluminum, and an alkaline earth metal,wettability of the metal silicon can be improved, whereby the strengthof the silicon carbide porous body is increased. In the method ofmanufacturing the silicon carbide porous body of the present invention,since the compounds containing silicon, aluminum, and an alkaline earthmetal are added so that the value obtained by converting the totalamount of aluminum contained in the compounds into the amount ofdialuminum trioxide (Al₂O₃) containing an equal amount of aluminum is5.0 mol % or more of the entire oxide phase formed after firing,formation of an alkaline earth metal silicate crystal phase can beinhibited. The compounds containing silicon, aluminum, and an alkalineearth metal used herein include silicon dioxide, Al, Ca, and the likecontained in the silicon carbide particles and the metal silicon asimpurities. Since the compounds are added so that the value obtained byconverting the total amount of aluminum into the amount of dialuminumtrioxide is 50 mol % or less of the entire oxide phase, the eutecticpoint of the ternary oxide phase can be sufficiently decreased, wherebythe strength can be improved. Moreover, since the honeycomb structureincludes the silicon carbide porous body of the present invention, theacid resistance of the honeycomb structure is improved, wherebycontamination of a solution used when causing the honeycomb structure tocarry a catalyst can be inhibited.

Embodiments of the present invention are described below. However, thepresent invention is not limited to the following embodiments. Variousmodifications and improvements of design may be made based on knowledgeof a person skilled in the art without departing from the scope andspirit of the present invention.

FIG. 1 is an enlarged cross-sectional diagram of a part of a crosssection cut at an arbitrary surface showing one embodiment of thesilicon carbide porous body of the present invention.

As shown in FIG. 1, a silicon carbide porous body 1 in this embodiment,in which silicon carbide particles 2 as an aggregate and metal silicon 3as a bonding material are bonded so that pores 5 are formed between thesilicon carbide particles 2, includes an oxide phase 4 in at least apart of each pore 5. It suffices that the oxide phase 4 be included inat least a part of each pore 5. As shown in FIG. 1, it is preferablethat the oxide phase 4 be disposed on the surface of the silicon carbideparticles 2 and/or the surface of the metal silicon 3 in the shape of afilm inside the pore formed by the silicon carbide particles 2 and themetal silicon 3. The oxide phase 4 may not be disposed in the shape of afilm, and the minute pores 5 may be filled with the oxide phase 4, forexample. However, it is preferable that the average pore size and theporosity be not too small. In this embodiment, the oxide phase 4includes silicon dioxide (SiO₂), dialuminum trioxide (Al₂O₃), andstrontium oxide (SrO), and does not substantially include an alkalineearth metal silicate crystal phase (SrSiO₃ or the like). Whether or notthe oxide phase 4 substantially includes an alkaline earth metalsilicate crystal phase can be confirmed by identifying the crystal phaseof the oxide phase 4 (oxide crystal phase) using X-ray diffraction(presence or absence of X-ray diffraction of SrSiO₃ or the like). Thestatement “does not substantially include an alkaline earth metalsilicate crystal phase” means that the X-ray diffraction pattern of analkaline earth metal silicate (SrSiO₃ or the like) crystal phase is notconfirmed in high-resolution X-ray diffraction measurement, for example.The statement “does not substantially include an alkaline earth metalsilicate crystal phase” also includes the case where, even if the oxidephase 4 includes silicon dioxide (SiO₂) and strontium oxide (SrO) in thecomposition, the oxide phase 4 forms an amorphous oxide phase and/or anoxide crystal phase including aluminum other than an alkaline earthmetal silicate, such as strontium feldspar (SrAl₂Si₂O₈), due toinclusion of dialuminum trioxide (Al₂O₃). It suffices that the silicondioxide be an oxide of silicon and the dialuminum trioxide be an oxideof aluminum. It suffices that the strontium oxide be an oxide of analkaline earth metal. In more detail, it suffices that the strontiumoxide be an oxide of at least one alkaline earth metal selected from thegroup consisting of magnesium, calcium, strontium, and barium (e.g. MgO,CaO, or the like). In this case, the alkaline earth metal may be onlyone type, or different types of alkaline earth metals may be mixed. Thealkaline earth metal silicate crystal phase refers to any alkaline earthmetal silicate which does not contain aluminum, such as CaSiO₃, Ca₂SiO₄,or SrSiO₃.

In the present invention, whether or not the oxide phase 4 substantiallyincludes the alkaline earth metal silicate crystal phase may beconfirmed by measuring the X-ray diffraction pattern of a specimen usinga high-resolution powder X-ray diffractometer (HR-XRD) “RINT-2500”manufactured by Rigaku Corporation under predetermined conditions(X-rays: Cu Kα1, tube voltage: 50 kV, tube current: 300 mA, counter:scintillation counter, goniometer: RINT 2000 wide-angle goniometer,attachment: ASC-6S, counter monochromator: full-automatic monochromator,divergence slit: 1 deg, scattering slit: 1 deg, receiving slit: 0.3 mm,scan mode: continuous, scan speed: 2°/min, scan step: 0.02°, scan axis:2θ/θ), and confirming whether or not the diffraction line of an alkalineearth metal silicate crystal phase is observed in the resultingdiffraction pattern, for example. Even if a trace of an alkaline earthmetal silicate crystal phase in an amount equal to or less than thedetection limit of the above device is included, it is determined that“the oxide phase 4 does not substantially include an alkaline earthmetal silicate crystal phase” when the X-ray diffraction pattern of analkaline earth metal silicate crystal phase is not observed.

The silicon carbide porous body 1 in this embodiment configured asdescribed above is used as a filter for carrying a catalyst componentand removing a toxic substance or the like.

Since the silicon carbide porous body 1 in this embodiment includes theoxide phase 4 in at least a part of each pore 5, and the oxide phase 4includes oxides of silicon, aluminum and an alkaline earth metal, andthe oxide phase 4 does not substantially include an alkaline earth metalsilicate crystal phase, the oxide phase is not dissolved in an acid(particularly acetic acid) used for loading a catalyst on the body 1 toexhibit improved acid resistance, whereby contamination of a solutionused for loading a catalyst on the body 1 is inhibited. Specifically,when a catalyst is loaded on the silicon carbide porous body 1, thecatalyst is dissolved in an acid (acetic acid or the like), and thesolution is applied to the silicon carbide porous body 1, for example.In this case, since an alkaline earth metal silicate crystal phase doesnot substantially exist in the silicon carbide porous body 1, the oxidephase is not dissolved in the acid (acetic acid or the like), wherebycorrosion of the silicon carbide porous body 1 is inhibited.

Since the silicon carbide porous body 1 in this embodiment includes thesilicon carbide particles 2 as an aggregate and the metal silicon 3, thesilicon carbide porous body 1 can be sintered at a relatively low firingtemperature during the manufacturing, whereby manufacturing cost isreduced and manufacturing yield is improved. Since the metal silicon 3is used to bond the silicon carbide particles 2 which are refractoryparticles, the silicon carbide porous body 1 exhibits high thermalconductivity. Therefore, when the silicon carbide porous body 1 is usedfor a DPF, a local increase in temperature which may cause damage to thefilter rarely occurs even if deposited particulates are burned forfilter regeneration.

Since the silicon carbide porous body 1 includes the oxide phase 4including oxides of silicon, aluminum, and an alkaline earth metal in atleast a part of each pore 5, oxidation and decomposition of the siliconcarbide particles 2 and the metal silicon 3 are inhibited even if thesilicon carbide porous body 1 is subjected to a high temperature in alow oxygen atmosphere encountering when used as a DPF. Specifically,since the silicon carbide porous body in this embodiment exhibitsexcellent strength, oxidation resistance, and thermal shock resistance,generation of heat due to oxidation of the silicon carbide and the metalsilicon during filter regeneration rarely occurs, whereby the filter isdamaged to only a small extent. It is preferable that the oxide phase 4be provided on the surface of the silicon carbide particles 2 and/or thesurface of the metal silicon 3. This further improves strength,oxidation resistance, and thermal shock resistance.

In the silicon carbide porous body 1 in this embodiment, the dialuminumtrioxide is included in the oxide phase 4 in an amount of preferably 5.0to 50.0 mol %, still more preferably 7.0 to 40.0 mol %, and particularlypreferably 8.0 to 35.0 mol % of the entire oxide phase in molar ratio.If the content of the dialuminum trioxide is 5.0 mol % or more,formation of an alkaline earth metal silicate formed from the alkalineearth metal oxide and the silicon dioxide included in the oxide phase 4can be inhibited (amorphized). If the content of the dialuminum trioxideis equal to or greater than a specific value, a part or the entirety ofthe oxide phase 4 may form a crystal phase including dialuminum trioxidein the crystal structure, such as a cordierite crystal, anorthitecrystal, or strontium feldspar (SrAl₂Si₂O₈). Since the amorphous phaseincluding aluminum and the crystal phase including dialuminum trioxidein the crystal structure, which are formed in place of an alkaline earthmetal silicate crystal phase, are not dissolved in an acid (particularlyacetic acid) used for loading a catalyst on the body 1, the acid (aceticacid) resistance of the silicon carbide porous body 1 is improved. Inorder to melt and remove the oxide film (SiO₂ or the like) on thesurfaces of the silicon carbide particles and the metal silicon toimprove wettability of the metal silicon by reducing the eutectic pointof the oxide phase 4 made up of oxides of silicon, aluminum, and analkaline earth metal as a ternary oxide phase, the dialuminum trioxideis included in the oxide phase 4 in an amount of preferably 5.0 to 50.0mol %, still more preferably 7.0 to 40.0 mol %, and particularlypreferably 8.0 to 35.0 mol %.

If the content of the dialuminum trioxide is less than 5.0 mol %, thedialuminum trioxide may not be sufficiently diffused in the oxide phase4, whereby formation of an alkaline earth metal silicate crystal phasemay not be prevented. If the content of the dialuminum trioxide isgreater than 50.0 mol %, the eutectic point of the oxide phase 4 may notbe sufficiently decreased, whereby wettability between the siliconcarbide particles and the metal silicon may be decreased.

It is preferable that the content of the silicon dioxide in the oxidephase 4 be 10.0 to 70.0 mol % from the viewpoint of decreasing theeutectic point of the oxide phase 4. If the content is less than 10 mol% or greater than 70.0 mol %, the eutectic point of the oxide phase 4may not be sufficiently decreased. It is preferable that the content ofthe alkaline earth metal oxide in the oxide phase 4 be 10.0 to 70.0 mol% from the viewpoint of decreasing the eutectic point of the oxide phase4. If the content is less than 10 mol % or greater than 70.0 mol %, theeutectic point of the oxide phase 4 may not be sufficiently decreased.

The ratio of the amount of the silicon dioxide, dialuminum trioxide, andalkaline earth metal oxide in the oxide phase 4 may be calculated fromvalues obtained by converting each element contained in the compoundscontaining silicon, aluminum, and alkaline earth metal to be added andaluminum, calcium, and the like contained in the metal silicon asimpurities into respective oxides, and values obtained by converting theamount of silicon dioxide contained in the oxide film on the surface ofthe silicon carbide particles and/or the metal silicon from the amountof oxygen determined by chemically analyzing the raw material powder.These values may also be calculated by determining the amount of eachelement by measuring characteristic X-rays specific to silicon,aluminum, and alkaline earth metal by EDS point analysis or the like ofthe oxide phase 4 present on the ground surface of the resulting siliconcarbide porous body 1, or determining the amount of each element bypredetermined chemical analysis or the like. The measurement method isnot limited to those described above.

One embodiment of the method of manufacturing the silicon carbide porousbody of the present invention is described below.

In the method of manufacturing the silicon carbide porous body in thisembodiment, predetermined amounts of compounds containing silicon,aluminum, and an alkaline earth metal are added to silicon carbideparticles and metal silicon. A pore-forming agent or the like isarbitrarily added to the mixture to obtain a raw material mixture.

After the addition of a forming agent such as an organic binder to theraw material mixture, as required, the components are mixed to obtainforming clay.

The compounds containing silicon, aluminum, and alkaline earth metal inthe clay are added so that the content of dialuminum trioxide is 5.0 to50.0 mol % of the entire oxide phase including oxides of silicon,aluminum, and an alkaline earth metal and provided on at least a part ofthe surface of the silicon carbide particles and/or the metal silicon ofthe silicon carbide porous body obtained after firing. The amount ofaddition may be calculated from values obtained by converting eachelement contained in the compounds containing silicon, aluminum, andalkaline earth metal to be added and aluminum, calcium, and the likecontained in the metal silicon as impurities into respective oxides, andvalues obtained by converting the amount of silicon dioxide contained inthe oxide film on the surface of the silicon carbide particles and/orthe metal silicon from the amount of oxygen determined by chemicallyanalyzing the raw material powder. The alkaline earth metal contained inthe compound to be added is preferably contained in the form of amonoxide or carbonate, such as strontium oxide (SrO) or strontiumcarbonate (SrCO₃), since the oxide phase can be efficiently formed andthese compounds are easily available and handled. The aluminum ispreferably contained in the form of dialuminum trioxide (Al₂O₃) or metalaluminum (Al). In this case, metal aluminum may be contained asimpurities in the metal silicon. The silicon is preferably contained inthe form of silicon dioxide (SiO₂) or colloidal silica. In this case,silicon dioxide may be contained as an oxide film which covers thesurfaces of the silicon carbide particles and the metal silicon. As thepore-forming agent, an organic pore-forming agent such as starch or afoamed resin may be used.

The resulting clay is formed into a predetermined shape such as ahoneycomb shape. The resulting formed product is calcinated to removethe organic binder in the formed product (debinding), and then fired toobtain a silicon carbide porous body.

In the method of manufacturing the silicon carbide porous body in thisembodiment, since the content of the dialuminum trioxide after aluminumcontained in the compound added and aluminum contained in the metalsilicon are converted into dialuminum trioxide is adjusted to 5.0 to50.0 mol % of the entire oxide phase, an amorphous phase includingaluminum and/or a crystal phase including dialuminum trioxide in thecrystal structure is formed in the oxide phase, whereby formation of analkaline earth metal silicate crystal phase can be inhibited. Since theamorphous phase including aluminum and the crystal phase includingdialuminum trioxide in the crystal structure are not dissolved in anacid (particularly acetic acid) used for loading a catalyst, a siliconcarbide porous body having improved acid (acetic acid) resistance can bemanufactured.

In the method of manufacturing the silicon carbide porous body in thisembodiment, it is preferable to perform calcinating at a temperaturelower than the melting temperature of the metal silicon. In more detail,the formed product may be held at a predetermined temperature of about150 to 700° C., or the formed product may be calcinated in apredetermined temperature range at a temperature rise rate as low as 50°C./hr or less. In the case of holding the formed product at apredetermined temperature, the formed product may be held at only onetemperature level or a plurality of temperature levels depending on thetype and the amount of the organic binder. In the case of holding theformed product at a plurality of temperature levels, the holding time ateach temperature level may be the same or different. In the case ofdecreasing the temperature rise rate, the temperature rise rate may bedecreased within only one temperature range or over a plurality oftemperature ranges. In the case of decreasing the temperature rise rateover a plurality of temperature ranges, the temperature rise rate ineach temperature range may be the same or different.

It is necessary to soften the metal silicon during firing in order tocause the resulting silicon carbide material to have a porous structurein which the refractory particles included therein are bonded throughthe metal silicon. Since the melting point of the metal silicon is 1410°C., it is preferable to set the firing temperature at 1410° C. or more.An optimum firing temperature is determined depending on themicrostructure and the characteristic values. If the firing temperatureexceeds 1600° C., bonding through the metal silicon becomes difficultdue to progress of vaporization of the metal silicon. Therefore, thefiring temperature is preferably 1410 to 1600° C., and still morepreferably 1420 to 1580° C.

One embodiment of the honeycomb structure of the present invention isdescribed below.

The honeycomb structure in this embodiment is a structure formed of thesilicon carbide porous body of the present invention, and including aplurality of cells functioning as fluid channels. The shape of thehoneycomb structure is not particularly limited. For example, thehoneycomb structure has a columnar structure. The cross-sectional shapecut at a plane perpendicular to the axial direction of the columnarstructure is polygonal (such as quadrangular), circular, elliptical,oval, unsymmetrical, or the like. The cross-sectional shape of the cellis not particularly limited. The cross-sectional shape of the cell istriangular, quadrangular, hexagonal, circular, or the like. The densityof the cells functioning as fluid channels is not particularly limited.An optimum cell density may be selected depending on the application.The honeycomb structure in this embodiment exhibits excellent acid(acetic acid) resistance, oxidation resistance, particulate reactionresistance, and thermal shock resistance due to the characteristics ofthe silicon carbide porous body of the present invention as theconstituent material. The honeycomb structure of the present inventionmay be used under high space velocity (SV) conditions as a DPF, catalystregeneration DPF, catalyst carrier, or the like.

In the method of manufacturing the honeycomb structure in thisembodiment, clay is formed into a honeycomb shape by extrusion or thelike in the above-described embodiment of the method of manufacturingthe silicon carbide porous body of the present invention when formingthe clay into a predetermined shape. The resulting formed product iscalcinated and then fired to obtain a honeycomb structure formed of thesilicon carbide porous body.

EXAMPLE

Examples of the present invention are described below. However, thepresent invention is not limited to the following examples.

Examples 1 to 4 and Comparative Examples 1 to 4

Silicon carbide powder (silicon carbide particles) with an averageparticle diameter of 33 μm and metal silicon powder with an averageparticle diameter of 5 μm were mixed so that the ratio of the mass ofthe silicon carbide powder to the total value of the mass of the siliconcarbide powder and the mass of the metal silicon powder was 80 mass %.After the addition of compounds containing silicon, aluminum, and analkaline earth metal, the components were mixed to obtain a raw materialmixture.

After the addition of 6 parts by mass of methyl cellulose as an organicbinder, 2.5 parts by mass of a surfactant, and 24 parts by mass of waterto 100 parts by mass of the raw material mixture, the components weremixed. The mixture was then kneaded for 30 minutes using a vacuumkneader to obtain forming clay. Table 1 shows the ratio of eachcomponent (SiO₂ content, Al₂O₃ content, and alkaline earth metal oxide(XO) content) calculated from values obtained by converting eachcomponent (oxide phase component) in the oxide phase obtained afterfiring, each element contained in the compounds containing silicon,aluminum, and alkaline earth metal, and aluminum, calcium, and the likecontained in the metal silicon as impurities into respective oxides, andvalues obtained by converting the amount of silicon dioxide contained inthe oxide film on the surface of the silicon carbide particles and/orthe metal silicon from the amount of oxygen determined by chemicallyanalyzing the raw material powder. TABLE 1 Crystal type of oxide Oxidephase component Al₂O₃ content (mol %) SiO₂ content (mol %) XO content(mol %) phase Example 1 SrO, Al₂O₃, SiO₂ 8.2 28.4 63.4 Amorphous Example2 SrO, Al₂O₃, SiO₂ 35.1 26.1 38.8 SrAl₂Si₂O₈ Example 3 MgO, Al₂O₃, SiO₂41.5 23.2 35.3 Cordierite Example 4 CaO, Al₂O₃, SiO₂ 32.4 18 49.6Anorthite Comparative MgO, Al₂O₃, SiO₂ 4.8 37.8 57.4 Forsterite(Mg₂SiO₄) Example 1 Comparative CaO, Al₂O₃, SiO₂ 3.5 25.7 70.8 Calciumsilicate Example 2 (CaSiO₃) Comparative SrO, Al₂O₃, SiO₂ 4 29.7 66.3Strontium silicate Example 3 (SrSiO₃) Comparative SrO, Al₂O₃, SiO₂ 55.314.5 30.2 SrAl₂Si₂O₈ Example 4X: Alkaline earth metal (Sr, Mg, or Ca)

The resulting clay was formed into a honeycomb shape having an outerdiameter of 45 mm, a length of 120 mm, a partition wall thickness of0.43 mm, and a cell density of 100 cells/square inch (16 cells/cm²). Theresulting formed product was calcinated at 500° C. for five hours toremove the organic binder in the formed product (debinding), and thenfired at 1450° C. for two hours in a non-oxidizing atmosphere to obtaina silicon carbide porous body (Examples 1 to 4 and Comparative Examples1 to 4). The resulting silicon carbide porous body was subjected to thefollowing evaluations.

(Crystal Type of Oxide Phase)

The crystallization state and the type of the crystal (crystallizedsubstance) of the oxide phase (crystal type of oxide phase) of theresulting silicon carbide porous body (Examples 1 to 4 and ComparativeExamples 1 to 4) were identified using X-ray diffraction. The resultsare shown in Table 1.

Identification Method For Crystal Type of Oxide Phase:

The X-ray diffraction pattern of each silicon carbide porous bodyobtained by using a predetermined measurement method was compared withthe X-ray diffraction pattern of a known oxide crystal to determine thecrystal type of the oxide phase. In this case, it suffices that only asmall peak be confirmed as the X-ray diffraction pattern of the oxidecrystal. A phase for which the X-ray diffraction pattern originating inthe oxide crystal phase could not be confirmed or a phase for which abroad diffraction originating in an amorphous glass component wasconfirmed at about θ=20 to 30° was determined as an amorphous phase. Inthis case, whether or not the amorphous phase includes aluminum may beconfirmed by measuring characteristic X-rays specific to aluminum by EDSpoint analysis or the like of the oxide phase present on the groundsurface of the resulting silicon carbide porous body. In the case wherean oxide crystal phase and an amorphous phase are formed in a mixedstate, the oxide crystal phase which could be identified was determinedas the crystal type of the oxide phase for convenience (amorphous phaseis not dissolved in acetic acid).

(X-ray Diffraction Measurement Method)

The above predetermined measurement method may be a method of measuringand analyzing the X-ray diffraction pattern of a specimen using ahigh-resolution powder X-ray diffractometer (HR-XRD) “RINT-2500”manufactured by Rigaku Corporation under predetermined conditions(X-rays: Cu Kα1, tube voltage: 50 kV, tube current: 300 mA, counter:scintillation counter, goniometer: RINT 2000 wide-angle goniometer,attachment: ASC-6S, counter monochromator: full-automatic monochromator,divergence slit: 1 deg, scattering slit: 1 deg, receiving slit: 0.3 mm,scan mode: continuous, scan speed: 2°/min, scan step: 0.02°, scan axis:2θ/θ). However, the measurement method is not limited to theabove-mentioned measurement method and measurement conditions. It ispreferable that the measurement device have a higher resolution.

(Porosity)

The porosity (%) of the resulting silicon carbide porous body (Examples1 to 4 and Comparative Examples 1 to 4) was measured using an Archimedesmethod. The results are shown in Table 2.

(Evaluation of Strength)

The strength of the resulting silicon carbide porous body (Examples 1 to4 and Comparative Examples 1 to 4) was measured using the followingmethod. The results are shown in Table 2.

Strength Measurement Method:

A four-point bending strength at room temperature was measured inaccordance with the method described in JIS R1601.

(Evaluation of Acetic Acid Resistance)

The acetic acid resistance of the resulting silicon carbide porous body(Examples 1 to 4 and Comparative Examples 1 to 4) was evaluated. Theresults are shown in Table 2.

Evaluation Method for Acetic Acid Resistance:

The silicon carbide porous body was immersed in a 10 mass % acetic acidaqueous solution for 30 minutes, and a change in mass before and afterimmersion was measured. The mass reduction rate (mass %) was calculatedusing the following equation (1), and taken as the index of the aceticacid resistance. The smaller the mass reduction rate, the more excellentthe acetic acid resistance. The results are shown in Table 2. A changein mass may be calculated by determining the quantity of silicate ionssuch as SiO₃ ²⁻ and SiO₄ ⁴⁻ and alkaline earth metal ions such as Sr²⁺dissolved in the acetic acid aqueous solution by chemical analysis afterthe test. The evaluation method for acetic acid resistance is notlimited to that described above.(Mass reduction rate)=((mass before immersion)−(mass afterimmersion))/(mass before immersion)×100   (1)Mass before immersion: dry mass before immersing silicon carbide porousbody in acetic acid aqueous solution

Mass after immersion: dry mass after immersing silicon carbide porousbody in acetic acid aqueous solution TABLE 2 Porosity Strength Aceticacid resistance (%) (MPa) (mass %) Example 1 44 30 0 Example 2 42 24 0Example 3 46 23 0 Example 4 43 28 0 Comparative Example 1 47 — 1.1Comparative Example 2 46 — 1.2 Comparative Example 3 45 — 1.5Comparative Example 4 45 15 0

As shown in Table 2, since formation of an alkaline earth metal silicatecrystal phase is inhibited when the content of dialuminum trioxide inthe oxide phase is 5.0 mol % or more, the oxide phase is not dissolvedin acetic acid. A silicon carbide porous body having high strength andexcellent acetic acid resistance is obtained when the content ofdialuminum trioxide in the oxide phase is 50 mol % or less.

Industrial Applicability

As described above, since the silicon carbide porous body of the presentinvention includes the oxide phase in at least a part of each pore, andthe oxide phase includes oxides of silicon, aluminum and an alkalineearth metal, and the oxide phase does not substantially include analkaline earth metal silicate, the oxide phase is not dissolved in anacid (particularly acetic acid) used for loading a catalyst on the bodyto exhibit improved acid resistance, whereby contamination of a solution(solution in which catalyst is dissolved in acid) used for loading acatalyst on the body is inhibited. According to the method ofmanufacturing the silicon carbide porous body of the present invention,since the compounds containing silicon, aluminum, and an alkaline earthmetal are added so that the value obtained by converting the totalamount of aluminum contained in the compounds into the amount ofdialuminum trioxide (Al₂O₃) containing an equal amount of aluminum is5.0 mol % or more of the entire oxide phase formed after firing,formation of an alkaline earth metal silicate crystal phase can beinhibited. Since the compounds are added so that the value obtained byconverting the total amount of aluminum into the amount of dialuminumtrioxide is 50 mol % or less of the entire oxide phase, the eutecticpoint of the ternary oxide phase can be sufficiently decreased, wherebythe strength can be improved. Since the honeycomb structure of thepresent invention includes the silicon carbide porous body, the acidresistance of the honeycomb structure is improved, whereby contaminationof a solution used when causing the honeycomb structure to carry acatalyst can be inhibited.

1-8. (canceled)
 9. A silicon carbide porous body, in which siliconcarbide particles as an aggregate and metal silicon as a bondingmaterial are bonded so that pores are formed between the silicon carbideparticles, the silicon carbide porous body comprising an oxide phase inat least a part of each pore, wherein the oxide phase includes oxides ofsilicon, aluminum and an alkaline earth metal, and the oxide phase doesnot substantially include an alkaline earth metal silicate crystalphase, and wherein the oxide of aluminum is dialuminum trioxide (Al₂O₃),the oxide phase including the dialuminum trioxide in an amount of 5.0 to50.0 mol % of the entire oxide phase in molar ratio.
 10. The siliconcarbide porous body as defined in claim 9, wherein the oxide phase isprovided on a surface of the silicon carbide particles and/or a surfaceof the metal silicon.
 11. The silicon carbide porous body as defined inclaim 9, wherein the oxide of silicon is silicon dioxide (SiO₂), and theoxide of an alkaline earth metal is calcium oxide (CaO) or strontiumoxide (SrO).
 12. The silicon carbide porous body as defined in claim 10,wherein the oxide of silicon is silicon dioxide (SiO₂), and the oxide ofan alkaline earth metal is calcium oxide (CaO) or strontium oxide (SrO).13. The silicon carbide porous body as defined in claim 9, wherein theoxide phase includes silicon dioxide, dialuminum trioxide, and the oxideof an alkaline earth metal, the oxide phase being an amorphous phase ora crystal phase including dialuminum trioxide in a crystal structure.14. The silicon carbide porous body as defined in claim 13, wherein thecrystal phase includes cordierite, anorthite, or strontium feldspar(SrAl₂Si₂O₈).
 15. A honeycomb structure comprising a silicon carbideporous body, in which silicon carbide particles as an aggregate andmetal silicon as a bonding material are bonded so that pores are formedbetween the silicon carbide particles, the silicon carbide porous bodycomprising an oxide phase in at least a part of each pore, wherein theoxide phase includes oxides of silicon, aluminum and an alkaline earthmetal, and the oxide phase does not substantially include an alkalineearth metal silicate crystal phase, and wherein the oxide of aluminum isdialuminum trioxide (Al₂O₃), the oxide phase including the dialuminumtrioxide in an amount of 5.0 to 50.0 mol % of the entire oxide phase inmolar ratio.
 16. A method of manufacturing a silicon carbide porousbody, the method comprising: adding compounds containing silicon,aluminum, and an alkaline earth metal to silicon carbide particles andmetal silicon to obtain a raw material, forming the resulting rawmaterial into a predetermined shape, and calcinating and firing theresulting formed product to obtain a porous body including an oxidephase including oxides of silicon, aluminum, and an alkaline earth metalon at least a part of a surface of the silicon carbide particles and/orthe metal silicon and having a content of dialuminum trioxide (Al₂O₃) of5.0 to 50.0 mol % of the entire oxide phase in molar ratio.